Expert: Chanda Walker - 3/13/2009

Question
hello sir,you satisfied me by giving the answer that encouraged me to ask more.
Q1)What causes the fluttering of radio reception when an aeroplane flies overhead?
Q2)Why do radio radio waves diffract around buildings,although light wave do not?
Q3)What determines the desirable length and orientation of the "rabbit ears"antenna on a TV set?
Again I shall be very thankful to you.

Answer
I think the answer for Q1 and Q2 are the same.  I am a licensed radio amateur but I've never experienced or heard mention of airplanes causing radio reception problems.  But then again, I don't have many airplanes flying terribly near by.

My thoughts would be that it is caused by diffraction of the radio waves around the airplane.  

The reason radio waves diffract around buildings but light does not is simply due to the wavelength.  Light diffracts around things that are much smaller.  

Radio waves are measured in cm or meters depending on the band you are using.  Light waves are measured in microns.  There are 4-6 orders of magnitude difference between the sizes of these waves.  That means the things that can cause diffraction for radio waves can by 1000-100,000 times larger than the things that will cause diffraction in light waves.

Q3)  Rabbit ears are antennae.  Antennae need to a have an appropriate length determined by the wavelength of the signal you are trying to receive and the physics of the antenna.

I don't know much about rabbit ears themselves mostly because they won't work at my house--we have radiant barrier on the roof that prevents reception.  According to http://en.wikipedia.org/wiki/Dipole_antenna , rabbit ears are dipole antennae and so the length needs to be 1/2 the wavelength of the radiation you are trying to receive.

The wavelengths of TV in the US is about 4 meter down to about 0.5 meters which a very broad spectrum for one antenna.  Often rabbit ears have a UHF loop included to receive those higher frequencies (shorter wavelengths.)  That means the length should be something like 1 meter.  If you have a particular channel you are trying to receive you can calculate the wavelength and use 1/2 that length to improve the reception of that channel.

As far as the direction, in general, you want the ears to be pointing toward the signal origin.  By that I mean, adjust them so that neither of the two arms of the antenna are closer than the other to the origin of the signal.

However, radio waves reflect and diffract when inside buildings and off natural structures such as trees and mountains.  It is impossible for me to recommend an orientation for you.  The best modeling tools I've used don't really do an adequate job of predicting radio wave propagation when influenced by the surrounding environment.

All I can say is to try moving them around and see what happens.  Hopefully you'll find an orientation that will work!

Let me know if I can help further!